Render with artifacts and errors (sharp teeth in the shadow area) for basic paint texture
this looks like eevee. these things can happen in real time engines, they are called Shadow Acne. you seem to have a very pixelated shoadow map.
Look in to the shadow tab on the poperties bar to see what shadow map resolution you have (default 512px)

but they can have multiple solutions and causes, I suggest you read this article to understand more about them.

https://learnopengl.com/Advanced-Lighting/Shadows/Shadow-Mapping

but after i watched the video you sent in it seems more to be some sort of a bug with blender itself. using a different verson migth fix the issue